Double-beam type ultraviolet-visible spectrophotometers have been developed as a device for measuring the transmittance of specimens (e.g., see Patent Literature 1). FIG. 11 shows a schematic view of a double-beam type ultraviolet-visible spectrophotometer.
The ultraviolet-visible spectrophotometer 160 comprises: specimen cell 6; reference cell 8; light source unit 50 comprising spectrometer 2 and light source 1 that emits measurement light; photodetector 12; sector mirror (switching unit and light-shielding unit) 40; plurality of reflecting mirrors 3, 5, 7, 9, 10 and 11; enclosure 15 that does not allow light to pass through; index signal generator 20; analog/digital (A/D) converter 14; and computer 130 that controls the entire ultraviolet-visible spectrophotometer 160.
The specimen cell 6, reference cell 8, light source unit 50, photodetector 12, sector mirror (switching unit and light-shielding unit) 40 and reflecting mirrors 3, 5, 7, 9, 10 and 11 are disposed at predetermined positions inside enclosure 15.
A person performing an analysis can open a door provided in the enclosure 15 and replace the specimen cell 6 or reference cell 8 with a new specimen cell or a new reference cell.
Within light source unit 50, light emitted from light source 1 becomes incident to spectrometer 2 where monochromatic light (measurement light) of a desired wavelength λ, is extracted.
Sector mirror 40 alternately makes the monochromatic light into a specimen-side light beam LS or a reference-side light beam LR using a predetermined period. The sector mirror 40 is also provided with a light-shielding unit 41, which, as the sector mirror rotates, blocks the specimen-side light beam LS or reference-side light beam LR using a predetermined period and thereby creates alternating predetermined periods of specimen-side light beam LS incident duration and specimen-side light beam LS blocked duration and reference-side light beam LR incident duration and reference-side light beam LR blocked duration.
In synchrony with the rotation of sector mirror 40, which is rotatably driven at a predetermined speed, an index signal generator 20 generates index signal IDX at a rate of 1 pulse per rotation. As an example the rotational frequency of sector mirror 40 may be 50 Hz or 60 Hz in synchrony with the frequency of the power supply.
A computer 130 comprises a CPU (controller) 131 and memory (memory unit) 134. Further connected thereto are a display device 33 and an input device 32 comprising a keyboard, mouse and the like. To explain the functional blocks of the CPU 131, CPU 131 comprises a memory controller 31a for storing in memory 134 the output strength signal from a photodetector 12 and a computation controller 131b for calculating transmittance.
With an ultraviolet-visible spectrophotometer 160 such as the afore-described, light emitted by light source 1 becomes incident to spectrometer 2 where monochromatic light having a desired wavelength λ is extracted. The monochromatic light is sent to sector mirror 40 via reflecting mirror 3 and is then alternately separated by sector mirror 40 into specimen-side light beam LS and reference-side light beam LR.
First, the specimen-side light beam LS is irradiated onto specimen cell 6 via reflecting mirror 5, and the light that passes through specimen cell 6 is sent to the light-receiving surface of photodetector 12 via reflecting mirrors 9 and 11. The light that is sent to photodetector 12 is photo-electrically converted by photodetector 1, which outputs output strength signal S during the specimen-side incidence duration. Furthermore, because sector mirror 40 is provided with a light-shielding unit 41, which, together with the rotation of sector mirror 40, blocks specimen-side light beam LS using a predetermined period, the output strength signal from photodetector 12 that corresponds to light-shielding unit 41 becomes output strength signal DS during the specimen-side light-blocked duration. The output strength signals S and DS of photodetector 12 are sampled by A/D converter 14 using a predetermined time interval and are converted into digital voltage values (signal values).
The reference-side light beam LR is irradiated onto reference cell 8 via reflecting mirror 7, and the light that passes through reference cell 8 is sent to the light-receiving surface of photodetector 12 via reflecting mirror 10. The light that is sent to photodetector 12 is photo-electrically converted by photodetector 12 and is extracted as output strength signal R for the reference-side incidence duration. Furthermore, because sector mirror 40 is provided with a light-shielding unit 41, which, together with the rotation of sector mirror 40, blocks reference-side light beam LR using a predetermined period, the output strength signal from photodetector 12 that corresponds to light-shielding unit 41 becomes output strength signal DR during the reference-side light-blocked duration. The output strength signals R and DR of photodetector 12 are sampled by A/D converter 14 using a predetermined time interval and are converted to digital voltage values (signal values).
The memory controller 31a of computer 130 controls the storage in memory 134 of signal values (output strength signals S, DS, R and DR) from photodetector 12. When doing this, the signal values are stored in association with when the output strength signal was obtained from the photodetector 12, that is, whether during the period when light was blocked by light-shielding unit 41 or during the period when the optical path was switched by sector mirror 40.
FIG. 2 shows a timing chart for one rotation time span (which is defined as one period (N)) by sector mirror 40. FIG. 3 shows one example of the relationship between signal value (digital voltage value) and time for a plurality of periods.
During one period N, the photodetector outputs: output strength signal. S that corresponds to the incidence duration of specimen-side light beam LS; output strength signal DS that corresponds to the light-blocked duration of specimen-side light beam LS; output strength signal R that corresponds to the incidence duration of reference-side light beam LR; and output strength signal DR that corresponds to the light-blocked duration of reference-side light beam LR.
It should be noted that because the sampling period of A/D converter 14 is shorter than period N, a plurality (in this example, 6 each) of output data (“detection value data”) is output from A/D converter 14 for each signal that is output during one period N. This means that D1 through D6, D7 through D12, D13 through D18, and D19 through D24 are each detection value data that correspond to output strength signal S, output strength signal DS, output strength signal R and output strength signal DR, respectively.
Computation controller 131b performs a control so that transmittance is calculated using the equation (19) below based on output strength signal S, output strength signal DS, output strength signal R and output strength signal DR, which are stored as afore-described in memory 134.Transmittance (%)=((SN−DSN)/(RN−DRN))/Z×100  (19)
Z is the value of ((SN−DSN)/(RN−DRN)) stored in advance that was obtained by the measurement of a specimen (oftentimes water or air) used as control.
Specifically, for each period N, the average value for each of detection value data D4 through D6 corresponding to output strength signal SN, detection value data D10 through D12 corresponding to output strength signal DSN, detection value data D16 through D18 corresponding to output strength signal RN and detection value data D21 through D24 corresponding to output strength signal DRN is calculated. The average values are substituted into equation (19) to calculate the transmittance for the Nth period.